This invention relates to an encoder for use in encoding a speech signal into a plurality of excitation pulses which specify a sound source or a voice tract.
A conventional encoder of the type described is disclosed in U.S. Pat. No. 4,809,330 issued Feb. 28, 1989 to Tanaka et al and assigned to the instant assignee. In the Tanoka encoder, a speech signal is divided into a sequence of frames and each frame is encoded into a plurality of excitation pulses by the use of an autocorrelator and a cross-correlator. More particularly, a cross-correlation signal is derived not only from a current frame but also from a part of the next frame so as to remove interaction between the current and the next following frames. The excitation pulses for each frame are produced as a result of a pulse search operation carried out by the use of the above-mentioned cross-correlation signal for a pulse search duration longer than each frame.
With this structure, complicated compensation is required for every frame in connection with a previous frame. In addition, the pulse search operation is carried out for each frame over a pulse search duration longer than each frame. This makes the pulse search operation and a count of the excitation pulses difficult. Moreover, it is necessary to prepare a memory of a large memory capacity so as to store the speech signal because the pulse search operation is carried out over the pulse search duration which is typically two adjacent frames long.
Furthermore, it is to be noted that each value of the autocorrelation and the cross-correlation is usually fairly greater than unity when a fixed point calculation is carried out for calculating the autocorrelation and the cross-correlation. This results in expansion of a dynamic range for calculation of the autocorrelation and the cross-correlation and in degradation of both precision of calculation and quality of a reproduced voice.